This invention relates to printing, and in particular to registering printing plates on a plate handling machine such as a platesetter or other printing-related device.
In the printing industry it is common to wish to impart an image to a printing plate. Generally, one edge of the printing plate is used as a registration edge. The registration edge is used as a reference to properly align the printing plate on a plate handling machine such as a printing press or a platesetting machine. Typically, a plate is aligned on a drum of a plate handling machine by bringing its registration edge into contact with two registration pins or xe2x80x9cstopsxe2x80x9d. The drums of plate handling machines may have a number of stops projecting from them so that printing plates of various sizes can be registered on the drums.
Examples of plate handling machines equipped with such stops include the CREO(trademark) Computer-to-Plate (CTP) imaging systems which include a drum 8 with four stops 10A through 10D, as shown in FIG. 1. Drum 8 preferably comprises a right circular cylinder configured to rotate such that stops 10A through 10D are carried in a plate handling direction indicated by arrow 9. Stop 10A is positioned on a reference line 11 which is parallel to the axis of rotation of drum 8. Stops 10B-D are each offset from reference line 11 so that stops 10B through 10D lead stop 11A as drum 8 rotates. Throughout the description and claims, when a first object on a drum is said to xe2x80x9cleadxe2x80x9d a second object, it is to be understood that the first object passes a stationary line parallel to the axis of rotation of drum 8 before the second object as drum 8 rotates as indicated by arrow 9, provided that the first object is circumferentially separated from the second object by less than one half of a circumference of the drum. Stop 10D is positioned approximately on reference line 11 such that it leads reference line 11 by approximately 30 microns or less. Stop 10C is positioned to lead reference line 11 by approximately 250 microns. Stop 10B is positioned to lead reference line 11 by approximately 350 microns.
The arrangement of pins shown in FIG. 1 permits multiple different sizes of plates to be registered using the same arrangement of stops, as shown in FIG. 2. Small-sized plates 12A can be properly registered on stops 10A and 10B. Medium-sized plates 12B can be properly registered on stops 10A and 10C. Large-sized plates 12C can be properly registered on stops 10A and 10D. All plates are loaded so that they span between stop 10A and one of stops 10B, 10C and 10D depending upon the width of the plate. An edge detect strip (not shown) may be positioned parallel to reference line 11 on drum 8 to determine the lateral position of a plate.
Stops 10A through 10D are spread apart across the width of drum 8 in such a manner that there are no xe2x80x9cdead bandsxe2x80x9d between the maximum plate width for one pair of stops and the minimum plate width for the xe2x80x9cnextxe2x80x9d pair of stops. Thus, stops 10A through 10D allow for registration of plates with a continuous range of widths.
Despite the success of the systems shown in FIGS. 1 and 2, there is a need for systems capable of imaging a broader range of plate sizes.
The invention provides a surface for supporting a printing plate in a plate handling machine. The surface and the plate handling machine are movable in a plate handling direction relative to each other. The surface comprises a plurality of stops, the plurality of stops comprising a pair of central stops and at least two distal stops, wherein the pair of central stops are positioned along a reference line perpendicular to the plate handling direction and wherein each distal stop is offset from the reference line.
The plurality of stops may comprise a first group of stops and a second group of stops, each of the first and second groups of stops comprising one of the pair of central stops and at least one distal stop. The first and second groups of stops are preferably symmetrical with respect to a plane of symmetry which is perpendicular to the reference line.
The surface may be located on a drum connected to a drive mechanism configured to rotate the drum in a circumferential plate handling direction. The central stops preferably lead the distal stops as the drive mechanism rotates the drum in the circumferential plate handling direction.
The stops may project approximately 3 millimeters from the surface. The stops may alternatively be retractable from a position projecting approximately 3 millimeters from the surface to a position not extending from the surface.
The invention also provides a drum for a plate handling machine, the drum having a surface and configured to rotate about an axis of rotation. The drum comprises a plurality of stops on the surface of the drum, the plurality of stops comprising a pair of central stops and at least two distal stops. The pair of central stops are positioned along a reference line parallel to the axis of rotation of the drum and each distal stop is offset from the reference line. The distal stops are preferably offset from the reference line in the same direction. The distal stops are preferably offset from the reference line by a distance in a range of 5 to 1000 microns.
The drum may comprise a first stop positioned on the surface of the drum in a range of 80-90 centimeters in a first direction from a plane which bisects the drum perpendicular to the axis, a second stop positioned on the surface of the drum in a range of 60-70 centimeters in the first direction from the plane, a third stop positioned on the surface of the drum in a range of 30-40 centimeters in the first direction from the plane, a fourth stop positioned on the surface of the drum in a range of 30-40 centimeters in a second direction from the plane, wherein the second direction is opposite the first direction, a fifth stop positioned on the surface of the drum in a range of 60-70 centimeters in the second direction from the plane and a sixth stop positioned on the surface of the drum in a range of 80-90 centimeters in the second direction from the plane. The third and fourth stops are preferably positioned approximately on a reference line located on the surface of the drum parallel to the axis. The second and fifth stops are preferably offset by a first distance from the reference line in the circumferential direction, and the first and sixth stops are preferably offset by a second distance from the reference line in the circumferential direction, wherein the first distance is greater than the second distance.
The surface may comprise a first stop positioned on the surface in a range of 80-90 centimeters in a first lateral direction from a plane parallel to the plate handling direction which perpendicularly bisects the surface, a second stop positioned on the surface in a range of 60-70 centimeters in the first lateral direction from the plane, a third stop positioned on the surface in a range of 30-40 centimeters in the first lateral direction from the plane, a fourth stop positioned on the surface in a range of 30-40 centimeters in a second lateral direction from the plane, wherein the second lateral direction is opposite the first direction, a fifth stop positioned on the surface in a range of 60-70 centimeters in the second lateral direction from the plane and a sixth stop positioned on the surface in a range of 80-90 centimeters in the second lateral direction from the plane. The third and fourth stops are preferably positioned approximately on a reference line located on the surface and perpendicular to the plane. The second and fifth stops are preferably offset by a first distance from the reference line in the plate handling direction, and the first and sixth stops are preferably offset by a second distance from the reference line in the plate handling direction, wherein the first distance is greater than the second distance.
The invention also provides a method of mounting printing plates on the surface or drum disclosed herein. The method may comprise mounting a pair of smaller-sized plates on the stops, wherein each of the pair of smaller-sized plates spans one of the pair of central stops and one of the distal stops, removing the pair of smaller-sized plates from the surface and mounting a single larger-sized plate on the stops, wherein the single larger-sized plate spans the pair of central stops. The method may also comprise mounting a single larger-sized plate on the stops, wherein the single larger-sized plate spans the pair of central stops, removing single larger-sized plate from the surface and mounting a pair of smaller-sized plates on the stops, wherein each of the pair of smaller-sized plates spans one of the pair of central stops and one of the distal stops.